Re: [PATCH v4 bpf-next 2/3] bpf: Support "%c" in bpf_bprintf_prepare().

[Kuniyuki Iwashima <kuniyu@amazon.co.jp>]

From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Wed, 11 Aug 2021 14:15:50 -0700
> On Tue, Aug 10, 2021 at 2:29 AM Kuniyuki Iwashima <kuniyu@amazon.co.jp> wrote:
> >
> > /proc/net/unix uses "%c" to print a single-byte character to escape '\0' in
> > the name of the abstract UNIX domain socket.  The following selftest uses
> > it, so this patch adds support for "%c".  Note that it does not support
> > wide character ("%lc" and "%llc") for simplicity.
> >
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
> > ---
> >  kernel/bpf/helpers.c | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> >
> > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > index 15746f779fe1..6d3aaf94e9ac 100644
> > --- a/kernel/bpf/helpers.c
> > +++ b/kernel/bpf/helpers.c
> > @@ -907,6 +907,20 @@ int bpf_bprintf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
> >                         tmp_buf += err;
> >                         num_spec++;
> >
> > +                       continue;
> > +               } else if (fmt[i] == 'c') {
> 
> you are adding new features to printk-like helpers, please add
> corresponding tests as well. I'm particularly curious how something
> like "% 9c" (which is now allowed, along with a few other unusual
> combinations) will work.

I see. I'll add a test.
I'm now thinking of test like:
  1. pin the bpf prog that outputs "% 9c" and other format strings.
  2. read and validate it

Is there any related test ?
and is there other complicated fomat strings to test ?

Also, "% 9c" worked as is :)

---8<---
$ sudo ./tools/bpftool/bpftool iter pin ./bpf_iter_unix.o /sys/fs/bpf/unix
$ sudo cat /sys/fs/bpf/unix | head -n 1
        a
$ git diff
diff --git a/tools/testing/selftests/bpf/progs/bpf_iter_unix.c b/tools/testing/selftests/bpf/progs/bpf_iter_unix.c
index ad397e2962cf..8a7d5aa4c054 100644
--- a/tools/testing/selftests/bpf/progs/bpf_iter_unix.c
+++ b/tools/testing/selftests/bpf/progs/bpf_iter_unix.c
@@ -34,8 +34,10 @@ int dump_unix(struct bpf_iter__unix *ctx)
 
        seq = ctx->meta->seq;
        seq_num = ctx->meta->seq_num;
-       if (seq_num == 0)
+       if (seq_num == 0) {
+               BPF_SEQ_PRINTF(seq, "% 9c\n", 'a');
                BPF_SEQ_PRINTF(seq, "Num               RefCount Protocol Flags    Type St Inode    Path\n");
+       }
 
        BPF_SEQ_PRINTF(seq, "%pK: %08X %08X %08X %04X %02X %8lu",
                       unix_sk,
---8<---



> 
> > +                       if (!tmp_buf)
> > +                               goto nocopy_fmt;
> > +
> > +                       if (tmp_buf_end == tmp_buf) {
> > +                               err = -ENOSPC;
> > +                               goto out;
> > +                       }
> > +
> > +                       *tmp_buf = raw_args[num_spec];
> > +                       tmp_buf++;
> > +                       num_spec++;
> > +
> >                         continue;
> >                 }
> >
> > --
> > 2.30.2